When you go to add members to a S&C role, and you search for users, you need to enter the First Word of the user's DisplayName, otherwise you don't get results. I tried this in different tenants.
This is a very odd behaviour. If you enter user's email or UPN, you don't get results. If you enter the Name and the DiaplayName starts by Surname, you don't get results, and thus you may think there's something wrong with that user. But no, it's just the search only looking for the first word in DisplayName. Please widen the search scope for that searchbox.
The way I understand it, it's a filter based on the data shown in the SCC UI, not an actual search on the backend. But I totally agree, and it's so annoying to have different methods used in different parts of the service.